As a light source for a backlight unit of a liquid crystal display device, a white LED lamp, which emits white light, has been increasingly adopted instead of a conventional CCFL (cold cathode fluorescent lamp). In contrast with the conventional CCFL, the white LED lamp is advantageous in, for example, that no mercury is used therein, that it does not need to be provided with a high voltage circuit for electric discharge nor means for cutting noise from the circuit, and that it is able to be made compact easily.
On the other hand, in a liquid crystal display device that uses a conventional CCFL as a light source, heat generated by the CCFL is used to warm a liquid crystal panel. By warming the liquid crystal panel, it is possible to increase the liquid crystal response speed, to thereby reduce degradation of image display quality. However, the white LED lamp generates less heat than the CCFL, and thus, if the white LED lamp is used as a light source, it is sometimes difficult to sufficiently warm the liquid crystal panel. In such a case, the liquid crystal response speed is degraded, which may invite degradation of the image display quality.
In a case where the white LED lamp is used as a light source for an edge light type backlight, the white LED lamp is arranged at a periphery of the liquid crystal panel. In particular, in a case of a liquid crystal display device using a large-size liquid crystal panel, the liquid crystal panel is prone to in-plane temperature distribution. This invites distribution of liquid crystal response speed in the plane of the liquid crystal panel. In a case where the white LED lamp is used in such an edge light type liquid crystal display device, insufficient warming of the liquid crystal panel and occurrence of the distribution of the response speed may degrade the display quality of images still more than in a case where the white LED lamp is used as a light source in a direct type liquid crystal display device.
Examples of a method for reducing degradation of the response speed caused by insufficient warming of liquid crystal include, for example, a method disclosed in JP-A-S58-129421, where a polarization plate with a transparent conductive film is put on a liquid crystal display element to be used as a heater to warm liquid crystal to thereby achieve a faster response.
Furthermore, in the case of a liquid crystal display device using a white LED lamp as a backlight light source, red chromaticity may be degraded due to light emission characteristics of the white LED lamp. To prevent this, there has been invented a method of reducing degradation of red chromaticity by using a dedicated color filter corresponding to the light emission characteristics of the white LED lamp.